1. Field of the Invention
The present invention relates to a positioning method for positioning a head, such as a magnetic head, an optical pickup, or the like, on a desired position of a target track of a disk (recording medium) with high precision by using a general adjustment actuator and a fine adjustment actuator. The present invention further relates to a disk apparatus using such a control method.
2. Description of the Prior Art
In recent years, disk apparatuses, such as magnetic disk apparatuses, and the like, have been improved rapidly by decreasing the size and increasing the capacity thereof. For example, the capacity of a magnetic disk apparatus has been increased by increasing the track density of the magnetic disk, and the track pitch will be further reduced in the future. Therefore, in order to record/reproduce data to/from a magnetic disk, it is necessary to precisely position a magnetic head on concentrical tracks formed at a small pitch.
Generally, a magnetic head is supported by a head supporting mechanism provided in a magnetic disk apparatus. FIG. 9 is a plan view showing an exemplary structure of a conventional head supporting mechanism. A magnetic head 102 for recording/reproducing data to/from a spinning magnetic disk 101 is supported at an end of a suspension arm (also referred to as “supporting arm”) 103. The other end of the suspension arm 103 is supported at an end of a carriage (also referred to as “base arm”) 104. The carriage 104 is rotatably supported by a rotational bearing 105 which is fixed to a housing (not shown) of the magnetic disk apparatus.
A coil 106, which is a constituent of a voice coil motor (VCM), is fixed at the other end of the carriage 104. A magnetic circuit including a magnet 107 is fixed to the housing. The magnetic circuit controls a magnetizing current flowing through the coil 106 so that the carriage 104 pivots about the rotational bearing 105. With such a mechanism, the magnetic head 102, which is supported at the one end of the suspension arm 103, moves substantially in a radial direction of the magnetic disk 101.
The magnetic disk 101 contains servo information which are recorded in advance thereon at a predetermined angular interval. The magnetic head 102 is positioned based on the servo information. Specifically, in the positioning process of the magnetic head 102, the magnetic head 102 reads the servo information to detect track position information of the magnetic head 102. The magnetic head 102 generates a position error signal which indicates the position error of the magnetic head 102 with respect to a target track. The magnetic head 102 is positioned such that the position error signal is minimized. Such a positioning process is performed at every sampling cycle which is determined based on the rotation speed of the magnetic disk 101 and the number of pieces of servo information recorded in one track (the number of servo sectors in one track).
In order to precisely control the position of the magnetic head 102, it is necessary to shorten the sampling cycle to increase the control frequency of a magnetic head positioning system. However, the head supporting mechanism shown in FIG. 9 may have a natural vibration mode of higher-order. In such a case, if the control frequency is increased for the purpose of increasing the positioning precision, the positioning system may become unstable due to the natural vibration. In the conventional head supporting mechanism structure where the carriage 104 is rotated by the voice coil motor (VCM) 106 in order to position the magnetic head 102, it is difficult to achieve higher positioning precision.
As a countermeasure to such a problem, there have been proposed many implementations of a so-called “dual stage actuator” technique, wherein a fine adjustment actuator is further incorporated in the head supporting system, and the fine adjustment actuator is used in conjunction with the voice coil motor in order to precisely position a magnetic head. For example, in a proposed method, a thin film piezoelectric element which is capable of causing a tiny displacement and has a high natural resonance point is used as a fine adjustment actuator (see Japanese Unexamined Patent Publications Nos. 2001-216748 and 2002-134807 (corresponding to U.S. Patent Application Publication No. 2001/0021086 A1)). Another proposed method employs a servo technique in which a fine adjustment actuator is used in conjunction with a voice coil motor in order to position a magnetic head (see Japanese Patent No. 3089709).
On the other hand, as the size of magnetic disk apparatuses has been decreased in recent years, disturbance such as the friction force in a rotational bearing supporting a head supporting mechanism or the elastic force of the flexible print circuit that connects the actuator with a circuit substrate, presents a factor that further deteriorates the positioning precision for the magnetic head. In view of this, there has been proposed a positioning method for improving the positioning precision by compensating for such disturbance using disturbance estimation means. The disturbance estimation means operates based on a head position signal, which is extracted from servo information recorded on a magnetic disk, and a driving signal of the voice coil motor (see Japanese Unexamined Patent Publication No. 9-231701).
Increasing the control frequency of the magnetic head positioning system such that the maximum quick-response performance is gained in the fine adjustment actuator is a critical factor in the achievement of precise positioning of the magnetic head with the above-described dual stage actuator arrangement. However, in order to increase the control frequency, it is necessary to shorten the sampling cycle of sampling servo information. To this end, it is necessary to increase the number of servo sectors such that more servo information are recorded on a magnetic disk. However, as more servo information are recorded, the user data area is accordingly decreased, resulting in a decrease in the data format efficiency.
For example, as shown in FIG. 10, in a disk 101, servo areas 110 containing servo information and user data areas 111 are provided at a predetermined angular interval. For example, the length Ws of each servo area 110 and the length Wd of each user data area 111 for a 3.5-inch disk and a 1.8-inch disk are shown in the table of FIG. 11. As seen from FIG. 11, the length Wd of the user data area 111 decreases as the disk diameter decreases, but the length Ws of the servo area 110 does not decrease even if the disk diameter is decreased. This is because at least a certain amount of area is necessary for recording the servo information, and the length Ws of the servo area 110 cannot be decreased in proportion to the disk diameter.
The ratio of the servo areas 110 to the total area of the disk 101 is represented as Ws/(Wd+Ws). Thus, in the case of the 3.5-inch disk, the ratio is 50/(578+50)=8%. On the other hand, in the case of the 1.8-inch disk, the ratio is 50/(264+50)=16%. Thus, the ratio of the servo areas 110 to the total disk area increases as the disk size is decreased, resulting in a decrease in the data format efficiency.
In the above positioning method wherein disturbance is compensated for, the disturbance which acts on the head supporting mechanism is estimated based on the head position signal obtained from the servo information and the driving signal of the voice coil motor in order to compensate for external force. However, the servo information can be obtained only at every sampling cycle. Since this positioning method depends on the servo information, the band in which the disturbance can be estimated is limited by the sampling cycle of the servo information. As a result, the external force is not appropriately compensated for.
The present invention was conceived for the purpose of overcoming the above problems. An objective of the present invention is to provide a method for precisely positioning a head without increasing the number of servo sectors, i.e., without decreasing the data format efficiency. Another objective of the present invention is to provide a disk apparatus using such a positioning method.